1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and more particularly to a rotary brush that is used for a charger for electrically charging a photosensitive image carrier or intermediate transfer medium, a cleaning device for removing residual toner from the surface of an photosensitive image carrier or an intermediate transfer medium, or another similar device.
2. Prior Art
In an electrophotographic image forming apparatus, rotary brushes are used for various purposes. For example, they are used for devices such as: a brush charger which electrically charges an photosensitive image carrier; a transfer brush which gives a carrying power or an electrostatic transfer field to a transfer rotor; and a cleaning brush which removes residual toner from the surface of a photosensitive image carrier. A rotary brush is located in contact with a members opposite to it, like a photosensitive image carrier or a transfer rotor (hereinafter referred to as an opposite member). Generally, in order to produce a larger charging effect or cleaning effect, the brush is rotated at a peripheral velocity different from the peripheral velocity of the opposite member. However, this peripheral velocity difference might cause abrasion not only of the rotary brush but also of the opposite member surface and as a consequence, such surface degradation might deteriorate the image quality and shorten the service life of the opposite member and the rotary brush, thus unfavorably affecting durability.
As a first solution to this problem, such a constitution has been proposed that the longitudinal tips of the rotary brush fibers are curled and the brush is arranged to stay off the opposite member while it is not rotating, and come into contact with the opposite member by its centrifugal force while it is rotating (Japanese Examined Patent Publication No. H4-62665). A second proposed constitution is such that the rotary brush is in contact with the opposite member while the case cover of the image forming apparatus is closed and becomes off the opposite member while the case cover is open (Japanese Laid-open Patent Publication No. H6-348105).
A third proposed constitution concerns a cleaning device having a rotary cleaning member which performs cleaning while in contact with an opposite member (photosensitive drum) (Japanese Laid-open Patent Publication No.2002-287599). The device has a drive means to rotate the photosensitive drum and the rotary cleaning member in the same direction with a peripheral velocity difference, and a switching means to transmit the driving force from the drive means to the rotary cleaning member and shut it off, and while the driving force is not being transmitted from the drive means to the rotary cleaning member, the rotary cleaning member is rotated passively by the photosensitive drum.
In the above first constitution, the amount of push for pressing the rotary brush against the opposite member is restricted and therefore it is difficult to assure a stable brushing effect. Regarding the above second constitution, it has been pointed out that the durability of the rotary brush and the opposite member may be improved but their structures are more complicated, resulting in a larger size apparatus.
In the third constitution, when the rotary cleaning member which is rotated passively in contact with and driven by the opposite member (photosensitive drum) is a rotary brush, if the conditions for the brush to be driven (rotated passively) are not properly set, a peripheral velocity difference may arise in the plane of contact between the opposite member and the rotary brush, and as a consequence, the effect of abrasion reduction will not be sufficient. In other words, although the rotary brush fiber tips are in point contact with the opposite member and the overall contact area is small, the brush fiber tip orientation and contact pressure have wide latitude and thus, unless the amount of push for pressing the rotary brush against the opposite member is properly set, the effect of abrasion reduction will not be sufficient. The present invention is intended to solve the above problem.